Ball valves, whether manually, hydraulically or electrically actuated, typically include a spherical ball entrapped between two seals secured within a valve body. An actuator, by means of a sealed stem extending from the ball, rotates the ball within the valve body between full open and full closed positions.
With typical ball valves, such as that shown in FIGS. 1A through 1D, the spherical ball has a uniform diameter bore extending through it, corresponding to similar passages extending through each end of the valve body. When in the full open position, the bores are aligned axially, thereby providing the least amount of resistance to fluid flow through the passages. By gradually turning the handle (or other actuator) attached to the stem, the exposed area of the ball and its mating seal resemble a small "nail" or moon shape that widens dramatically with each degree of movement of the handle, as shown particularly in FIGS. 1B and 1C. The resulting intersecting area of the two round passages creates an unpredictable and extremely sensitive "exposed" area to fluid flow that often makes ball valves difficult to set for a desired flow rate.
For example, ball valves are often used to balance hydronic (heating ventilating and air conditioning or "HVAC") closed loop systems. The ball valves are often provided with built-in venturi and/or orifices to allow precise measurement of the flow rate through the ball valve. However, the standard configuration of a ball valve may not facilitate precise setting because of the sensitivity by degree of handle movement. Thus, it may take several passes on either side of the final actuator setting before a desired flow rate is achieved.
To minimize this sensitivity problem, some ball valves have been provided with relatively small bore sizes to alleviate low flow settings. Although providing better volume control, these ball valves may create an undesirably high friction loss for systems that require high flow conditions when the valve is fully open.
Alternatively, the passage of the ball valve may be provided with alternative bore openings in an attempt to improve throttling and control. For example, Toyo Company of Japan distributes ball valves that have spherical balls with diamond-shaped bores. In addition, U.S. Pat. No. 5,593,135 issued to Lester et al. discloses a ball valve that has a plastic spherical ball with a cylindrical bore extending most of the way through the ball. At one end, however, the bore is partially closed by a throttling wall formed as part of the spherical ball. The throttling wall has an oblong opening therein for providing a more linear flow condition as the ball is rotated within the valve body.
Such ball valves, however, require uniquely cast or machined spherical balls for each desired flow condition or "C.sub.v offering." In addition, diamond-shaped or other special port openings may not provide linear or other desired volumetric flow control for certain applications. Further, the cost and difficulty in manufacturing such customized ball valves may be further exacerbated when they are machined or formed from conventional materials, such as brass or stainless steel.
Accordingly, there is a need for a ball valve that provides more precise volumetric flow control than conventional ball valves.
In addition, there is a need for a ball valve that may provide a variety of selectable flow conditions, and that may be competitively manufactured substantially without customized forming methods.